1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a shingled magnetic recording (SMR) HDD that compensates for the effect of far track erasure (FTE) on data tracks in the boundary regions of data bands.
2. Description of the Related Art
Magnetic recording disk drives that use “shingle writing”, also called “shingled recording” or “shingled magnetic recording” (SMR), have been proposed, for example as described in U.S. Pat. No. 6,185,063 B1 and U.S. Pat. No. 6,967,810 B2. In SMR, the write head, which is wider than the read head in the cross-track direction, writes magnetic transitions by making a plurality of consecutive circular paths that partially overlap. The non-overlapped portions of adjacent paths form the shingled data tracks, which are thus narrower than the width of the write head. The data is read back by the narrower read head. The narrower shingled data tracks thus allow for increased data density. The shingled data tracks are arranged on the disk as annular bands separated by annular inter-band gaps or guard bands.
The writing of data to an entire band may occur when new data from the host is stored in memory and then written to a band for the first time. It may also occur when a portion of the data in a band is modified, i.e., a “read-modify-write” operation in which all the corresponding data in a band is read and stored in memory, then a portion is modified with the host-provided new write data, and finally all the corresponding data is written data back to the band. The writing of data to an entire band or bands may also occur when a band or bands are “cleaned” or “de-fragmented” to reclaim free space, i.e., the data in one or more bands is read and stored in memory and then re-written to the same band or a new band.
A problem in both conventional HDDs and SMR HDDs is wide-area track erasure (WATER) or far track encroachment or erasure (FTE). The write field from the write head is wider than a data track so when the write head is writing to a track, the outer portions of the write field (called the fringe field) overlap onto tracks other than the track being written. Data degradation due to fringe fields is not limited to the tracks immediately adjacent the track being written, but can extend over a range of tracks relatively far from the track being written. This FTE is particularly noticeable with write heads that have side shields. FTE may not affect tracks symmetrically on both sides of the track being written. Tracks on one side may encounter more pronounced FTE effects due to the write head shield design or due to read-write head skew. FTE is described by Liu et al., “Characterization of Skip or Far Track Erasure in a Side Shield Design”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009, pp. 3660-3663.
In a SMR disk drive, FTE can occur on the tracks in the boundary region of a band, i.e., those tracks near an inter-band gap, when data is written to tracks in the boundary region of an adjacent band. In U.S. application Ser. Nos. 13/423,177 and 13/423,178, both filed Mar. 17, 2012 and assigned to the same assignee as this invention, a counter is maintained for each data band and when data is written to the boundary region of a band or to an on-disk cache located between two data bands, the counters associated with adjacent bands are incremented by a value related to the previously determined FTE effect. When the count for a band reaches a predetermined threshold, the data in that band is rewritten. The data is rewritten before the FTE effects can build up, so the reliability of the data is improved.
What is needed is a SMR HDD that compensates for the FTE effect on adjacent bands without the need to continually read data from and rewrite data to bands affected by FTE.